The present invention relates generally to combustion gas turbine engines (“gas turbines”), and more specifically to a rim cavity sealing systems and processes for the gas turbine engines.
During operation, because of the extreme temperatures of the hot-gas path, great care is taken to prevent components from reaching temperatures that would damage or degrade their operation or performance. One area that is particularly sensitive to extreme temperatures is the space that is inboard of the hot-gas path. This area, which is often referred to as the inner wheelspace or rim cavity of the turbine, contains the several turbine wheels or rotors onto which the rotating rotor blades are attached. While the rotor blades are designed to withstand the high temperatures of the hot-gas path, the rotors are not and, thus, it is necessary that the working fluid of the hot-gas path be prevented from flowing into the rim cavity. However, as will be appreciated, axial gaps necessarily exist between the rotating blades and the surrounding stationary parts, and it is through these gaps that the hot gases of the working fluid gains access to the internal regions. In addition, because of the way the engine warms up and differing thermal expansion coefficients, these gaps may widen and shrink depending on the way the engine is being operated. This variability in size makes the proper sealing of these gaps a difficult design challenge.
More specifically, gas turbines includes a turbine section with multiple rows of stator blades and rotor blades in which the stages of rotor blades rotate together around the stationary guide vanes of the stator blades. The stator blades and assemblies related thereto extend into a rim cavity formed between two stages of the rotor blades. Seals are formed between the inner shrouds of the rotor blades and the stator blades, and between the inboard surface of the stator blade diaphragm and the two rotor disk rim extensions. As will be appreciated, the hot gas flow pressure is higher on the forward side of the stator blades than on the aft side, and thus a pressure differential exists within the rim cavity. In the prior art, seals on the inboard surface of the stator diaphragm may be used to control of leakage flow across the row of stator blades. Additionally, knife edge seals may be used on the stator blade cover plate to produce a seal against the hot gas ingestion into the rim cavity. Hot gas ingestion into the rim cavity is prevented as much as possible because the rotor disks are made of relatively low temperature material than the airfoils. The high stresses operating on the rotor disks along with exposure to high temperatures will thermally weaken the rotor disk and shorten the life thereof. Purge cooling air discharge from the stator diaphragm has been used to purge the rim cavity of hot gas flow ingestion.
However, very little progress has been made in the control of rim cavity leakage flow so to reduce the usage of purge air. Difficulties regarding distribution of purge are result in inefficient usage, which, of course, comes at a cost. As will be appreciated, purging systems increase the manufacturing and maintenance cost of the engine, and are often inaccurate in terms of maintaining a desired level of pressure or outflow from the rim cavity. Further, purge flows adversely affect the performance and efficiency of the turbine engine. That is, increased levels of purge air reduce the output and efficiency of the engine. Hence, it is desirable that the usage of purge air be minimized. As a result, there is a continuing need for improved methods, systems and/or apparatus that better seal the gaps, trench cavities, and/or rim cavities from the hot gases of the flow path.